The use of zeolite-based adsorbents comprising at least faujasite (FAU) zeolites of X or Y type and comprising, besides sodium cations, barium, potassium or strontium ions, alone or as mixtures, for selectively adsorbing para-xylene in a mixture of aromatic hydrocarbons, is well known from the prior art.
U.S. Pat. No. 3,558,730, U.S. Pat. No. 3,558,732, U.S. Pat. No. 3,626,020 and U.S. Pat. No. 3,663,638 show that zeolite-based adsorbents comprising aluminosilicates based on sodium and barium (U.S. Pat. No. 3,960,774) or based on sodium, barium and potassium, are efficient for separating para-xylene present in C8 aromatic fractions (fractions comprising aromatic hydrocarbons containing 8 carbon atoms).
The adsorbents described in U.S. Pat. No. 3,878,127 are used as adsorption agents in liquid-phase processes, preferably of simulated counter-current type, similar to those described in U.S. Pat. No. 2,985,589 and which apply, inter alia, to C8 aromatic fractions.
Document U.S. Pat. No. 6,884,918 recommends a faujasite X with an Si/Al atomic ratio of between 1.15 and 1.5 exchanged with barium or with barium and potassium. Document U.S. Pat. No. 6,410,815 teaches that zeolite-based adsorbents as described in the prior art, but for which the faujasite has a low content of silica and has an Si/Al atomic ratio close to 1 (which will be referred to as LSX, the abbreviation for low-silica X), are advantageously used for separating para-xylene, especially when feedstocks rich in ethylbenzene need to be treated, due to the better selectivity of para-xylene towards this isomer relative to adsorbents based on zeolite X with an Si/Al atomic ratio of between 1.15 and 1.5.
In the patents listed above, the zeolite-based adsorbents are in the form of crystals in powder form or in the form of agglomerates predominantly consisting of zeolite powder and up to 20% by weight of inert binder.
The synthesis of FAU zeolites is usually performed by nucleation and crystallization of silicoaluminate gels. This synthesis leads to crystals (generally in powder form) whose use at the industrial scale is particularly difficult (substantial losses of feedstocks during the manipulations). Agglomerated forms of these crystals are then preferred, in the form of grains, strands and other agglomerates, these said forms possibly being obtained by extrusion, pelleting, atomization and other agglomeration techniques known to those skilled in the art. These agglomerates do not have the drawbacks inherent in pulverulent materials.
Agglomerates, whether they exist in the form of platelets, beads, extrudates or the like, generally consist of zeolite crystals, which constitute the active element (in the sense of adsorption) and an agglomeration binder. This agglomeration binder is intended to ensure the cohesion of the crystals to each other in the agglomerated structure, but must also give said agglomerates sufficient mechanical strength so as to prevent, or at the very least to minimize, the risks of fractures, splitting or breaks that may arise during their industrial uses during which the agglomerates are subjected to numerous constraints, such as vibrations, high and/or frequent pressure variations, movements and the like.
The preparation of these agglomerates is performed, for example, by pasting zeolite crystals in powder form with a clay paste, in proportions of the order of 80% to 90% by weight of zeolite powder for 20% to 10% by weight of binder, followed by forming into beads, platelets or extrudates, and heat treatment at high temperature for baking of the clay and reactivation of the zeolite, the cation exchange(s), for instance the exchange with barium and optionally with potassium, possibly being performed before and/or after the agglomeration of the pulverulent zeolite with the binder.
Zeolite-based agglomerates are obtained, the particle size of which is a few millimeters, or even of the order of a millimeter, and which, if the choice of the agglomeration binder and the granulation are made according to the rule book, have a satisfactory set of properties, in particular of porosity, mechanical strength and abrasion resistance. However, the adsorption properties of these agglomerates are obviously reduced relative to the starting active powder due to the presence of agglomeration binder which is inert with respect to adsorption.
Various means have already been proposed for overcoming this drawback of the agglomeration binder being inert as regards the adsorption performance, among which is the transformation of all or at least part of the agglomeration binder into zeolite that is active from the adsorption viewpoint. This operation is now well known to those skilled in the art, for example under the name “zeolitization”. To perform this operation readily, zeolitizable binders are used, usually belonging to the kaolinite family, and preferably precalcined at temperatures generally between 500° C. and 700° C.
Patent FR 2 925 366 describes a process for manufacturing LSX zeolite is agglomerates, with an Si/Al atomic ratio such that 1.00≦Si/Al≦1.15 exchanged with barium and optionally with barium and potassium, by agglomerating LSX zeolite crystals with a kaolin-based binder, followed by zeolitizing the binder by immersing the agglomerate in an alkaline liquor. After exchanging the cations of the zeolite with barium ions (and optionally potassium ions) and activation, the agglomerates thus obtained have, from the point of view of adsorption of the para-xylene contained in C8 aromatic fractions and of the mechanical strength, improved properties relative to adsorbents prepared from the same amount of LSX zeolite and binder, but whose binder is not zeolitized.
Besides high adsorption capacity and good selectivity properties in favour of the species to be separated from the reaction mixture, the adsorbent must have good mass transfer properties so as to ensure a sufficient number of theoretical plates to perform efficient separation of the species in admixture, as indicated by Ruthven in the book entitled Principles of Adsorption and Adsorption Processes, John Wiley & Sons, (1984), pages 326 and 407. Ruthven indicates (ibid., page 243) that, in the case of an agglomerated adsorbent, the global mass transfer depends on the sum of the intra-crystalline and inter-crystalline (between crystals) diffusional resistances.
The intra-crystalline diffusional resistance is proportional to the square of the diameters of the crystals and inverse or proportional to the intra-crystalline diffusivity of the molecules to be separated.
The inter-crystalline diffusional resistance (also known as the “macropore resistance”) is itself proportional to the square of the diameters of the agglomerates, inversely proportional to the porosity contained in the macropores and mesopores (i.e. the pores with an aperture greater than 2 nm) in the agglomerate, and inversely proportional to the diffusivity of the molecules to be separated in this porosity.
The size of the agglomerates is an important parameter during the use of the adsorbent in industrial application, since it determines the loss of feedstock in the industrial unit and the filling uniformity. The particle size distribution of the agglomerates must thus be narrow, and centred on number-mean diameters typically between 0.40 mm and 0.65 mm so as to avoid excessive losses of feedstock.
The porosity contained in the macropores and mesopores in the agglomerate (the inter-crystalline macroporosity and mesoporosity, respectively) may be increased by using pore-forming agents, for instance corn starch as recommended in document U.S. Pat. No. 8,283,274 for improving the mass transfer. However, this porosity is does not participate in the adsorption capacity and the improvement of the macropore mass transfer then takes place to the detriment of the volume-based adsorption capacity. Consequently, this route for improving the macropore mass transfer proves to be very limited.
To estimate the improvement in transfer kinetics, it is possible to use the plate theory described by Ruthven in Principles of Adsorption and Adsorption Processes, ibid., pages 248-250. This approach is based on the representation of a column by a finite number of ideally stirred hypothetical reactors (theoretical stages). The equivalent height of theoretical plates is a direct measurement of the axial dispersion and of the resistance to mass transfer of the system.
For a given zeolite-based structure, a given size of adsorbent and a given operating temperature, the diffusivities are set, and one of the means for improving the mass transfer consists in reducing the diameter of the crystals. A gain in global mass transfer will thus be obtained by reducing the size of the crystals.
A person skilled in the art will thus seek to minimize the diameter of the zeolite crystals in order to improve the mass transfer.
Patent CN 1 267 185 C thus claims adsorbents containing 90% to 95% of zeolite BaX or BaXK for separating para-xylene, in which the zeolite X crystals are between 0.1 μm and 0.4 μm in size, so as to improve the mass transfer performance. Similarly, the application US 2009/0326308 describes a process for separating xylene isomers, the performance of which is improved by using adsorbents based on zeolite X crystals less than 0.5 μm in size. Patent FR 2 925 366 describes adsorbents containing LSX zeolite crystals with a number-mean diameter of between 0.1 μm and 4.0 μm.
The Applicant has nevertheless observed that the synthesis, filtration, handling and agglomeration of zeolite crystals whose size is less than 0.5 μm involve burdensome, uneconomical processes which are thus difficult to render industrializable.
Furthermore, such adsorbents comprising crystals less than 0.5 μm in size also prove to be more fragile, and it then becomes necessary to increase the content of agglomeration binder in order to reinforce the cohesion of the crystals with each other in the adsorbent. However, increasing the content of agglomeration binder leads to densification of the adsorbents, which causes an increase in the macropore diffusional resistance. Thus, despite a reduced intra-crystalline diffusional resistance due to the decrease in the size of the crystals, the increase in macropore diffusional resistance on account of the densification of the adsorbent does not allow an improvement in the overall transfer. Moreover, increasing the binder content does not make it possible to obtain a good adsorption capacity.
There is consequently still a need for improved zeolite-based adsorbent materials prepared from FAU zeolite crystals of LSX type that are easy to manipulate industrially, and for which said crystals (or constituent crystalline elements) are advantageously greater than 0.5 μm in size, and which have an improved global mass transfer relative to the adsorbents of identical crystal size known in the prior art, while at the same time conserving a high adsorption capacity and high adsorption selectivities for para-xylene with respect to its isomers.
These improved adsorbents would thus be particularly suitable for the gas-phase or liquid-phase separation of xylene isomers.